Overwatering is a very common cause of yellowing leaves, as the plant is robbed of oxygen. Your tub may not be draining as well as you anticipate. I would check your soil moisture first on that side of the tub. If it is most than a little moist a few inches down, that is your culprit.
If you purchased new soil for the tubs, it is probably not nutrient deficiency, although that is a possibility. So are some plant diseases. If the soil is not -- and has not -- been overly wet, please post a picture of the leaves in focus, which will aid in further diagnosis.